As a cyclist my primary concern is diesel trucks with scant emissions controls belching particulate and other combustion byproducts. I see little movement to get these off the road.
There are people who install "delete-pipes" though, and they should be taken out back and made to inhale what they spew.
Big lifted trucks spouting thick black smoke from their exhaust that leave behind little ashy flecks.
We've got a little '81 tractor that's a 3-cyc 1.4L diesel and if the wind blows the wrong way you definitely don't want to inhale too deeply. I'd love to convert it to Tier IV but there just aren't kits made for it.